The marginal cost C (in dollars) of manufacturing x cell phones (in thousands) is given by

C(x) = 5x² + 200x + 4000

(a) How many cell phones should be manufactured to minimize the marginal cost?

(b) What is the minimum marginal cost?

Data from problem 77

The marginal cost of a product can be thought of as the cost of producing one additional unit of output. For example, if the marginal cost of producing the 50th product is $6.20, it cost $6.20 to increase production from 49 to 50 units of output. Suppose the marginal cost C (in dollars) to produce x thousand mp3 players is given by the function

C(x) = x² - 140x + 7400